A metal oxide, a metal salt, and a metal compound such as an organometallic compound are used in a wide range of field; especially microparticles thereof are a material which is used widely in such fields as abrasive, catalyst, cosmetics, electronic equipment, magnetic material, pigment, covering and semiconductor.
It is known that an intended property, stability, and so forth can be improved by doping a metal compound with a different element therefrom. A property of such a metal compound can be improved further by making it nanoparticles; and thus, a method for producing metal compound nanoparticles having a homogeneously controlled amount of a dopant element and also having a uniform particle diameter is eagerly wanted.
An illustrative example of a method for producing a metal oxide having a controlled amount of an dopant element includes; as described in Patent Document 1, a method comprising a step of filtering a separated substance obtained by pouring, into an excess alkali aqueous solution, a mixture solution obtained by dissolving into water and so forth a water-soluble metal salt to form a metal oxide and a water-soluble metal salt used as a metal ion for valency control or formation of a solid solution of the foregoing metal oxide and a step of burning the separated substance; and as described in Patent Document 2, a method comprising a step of contacting metal oxide microparticles with a solution containing a dopant element and a step of burning the metal oxide microparticles which is contacted with the solution containing a dopant element. However, when these methods are used, to make amount of a dopant element in each particle homogeneous is sometimes difficult. In addition, particles are readily coarsened; and thus, to produce metal oxide nanoparticles having uniform and good re-dispersibility has been difficult.
Coarsened metal oxide particles are usually crushed by mechanical abrasion or mechanical crushing treatment with such equipment as a ball mill and a bead mill as described in Patent Document 3. However, it is difficult to produce uniform nanoparticles from metal oxide particles which are produced by the method like this; and in addition, to obtain particles having a homogeneously controlled amount of a dopant element has been sometimes difficult when a doped metal oxide is crushed. In addition, because a strong crushing force is applied to particles (crystals), there has been a problem that expected properties such as semiconductor characteristics, transparency, spectroscopic characteristics, and durability are not expressed.
The problems as mentioned above appear similarly in a metal compound other than the metal oxide compound and a non-metallic substance. This applicant of the present invention provided a method for producing a metal compound by separating particles in a thin film fluid which flows between processing surfaces arranged opposite to each other, as described in Patent Document 4; but a method for producing a metal compound having a controlled amount of a dopant element was not specifically disclosed therein.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2010-17649
Patent Document 2: Japanese Patent Application Publication No. 2007-525397
Patent Document 3: Japanese Patent Laid-Open Publication No. 2009-74080
Patent Document 4: International Patent Laid-Open Publication No. WO2009/020188 (semiconductor application)